(a) Field of the Invention
The present invention relates to a data recording medium having a thin magnetic alloy film as a recording layer and, more particularly, to a data recording medium having an anticorrosion protective film on the recording layer.
(b) Description of the Prior Art
A magneto-optical recording medium is receiving a great deal of attention as an optical disk capable of data read, write and erase operations upon radiation of a light beam such as a laser beam. A conventional magneto-optical recording medium of this type has a structure wherein a thin magnetic alloy film having an easy axis of magnetization in a direction perpendicular to a film surface is formed on a substrate as a recording layer. When a portion of the recording layer is heated to near the Curie point upon irradiation with an optical beam modulated in accordance with a data signal, data is recorded in the form of inversion of magnetization. In the read mode, a light beam irradiates a local portion of the recording layer which is subjected to inversion of magnetization, and rotation (i.e., polar-Kerr effect) of a polarization plane of the beam reflected from the recording layer surface is utilized to read the data.
Thin amorphous films of rare earth metal-transition metal alloys such as TbFe, DyFe, GdTbFe, TbFeCo and GdTbFeCo (known as a Curie point recording medium), or such as GdFe, GdCo, TbCo and GdTbCo (known as a compensation temperature recording medium) are known as conventional magnetic recording layers of the magneto-optical medium. These films have large Kerr rotation angles .theta.K practically, and have a merit that a large uniform recording layer can be formed by using simple methods e.g. sputtering, vacuum evaporation. In addition, these films have high recording efficiency at a low optothermal energy level so that a laser diode can be used for the recording operation.
When such a thin amorphous film of a rare earth metal-transition metal alloy is used as the recording layer of the magneto-optical recording medium, it generally has poor anticorrosion properties, particularly in the case of Curie point recording medium, which is the largest obstacle to the practical application of the alloy film in the magneto-optical recording medium. For this reason, according to a conventional technique, a transparent dielectric film is formed on the recording medium so as to improve the anticorrosion properties of such a film. However, since a SiO.sub.2 or a SiO film used as a dielectric film is formed by RF sputtering or the like, defects such as pinholes are formed in the film due to contamination during sputtering. Therefore, when the recording medium is used for a long time, moisture in the air penetrates the defects and corrodes the recording layer. This corrosion phenomenon is more prominent in the case of Curie point recording medium. The dielectric film cannot effectively improve the anticorrosion properties of the recording film. In addition to the magneto-optical recording medium, even in the conventional normal magnetic recording media, especially those using a thin magnetic alloy film such as Co-Cr as a recording layer, a dielectric film must be formed as a protective film on the recording layer, thus presenting the same problem as described above.